The 2018 Geothermal Reservoir Stimulation in Espoo/Helsinki, Southern Finland: Seismic Network Anatomy and Data Features

Abstract A seismic network was installed in Helsinki, Finland to monitor the response to an ∼6-kilometer-deep geothermal stimulation experiment in 2018. We present initial results of multiple induced earthquake seismogram and ambient wavefield analyses. The used data are from parts of the borehole network deployed by the operating St1 Deep Heat Company, from surface broadband sensors and 100 geophones installed by the Institute of Seismology, University of Helsinki, and from Finnish National Seismic Network stations. Records collected in the urban environment contain many signals associated with anthropogenic activity. This results in time- and frequency-dependent variations of the signal-to-noise ratio of earthquake records from a 260-meter-deep borehole sensor compared to the combined signals of 24 collocated surface array sensors. Manual relocations of ∼500 events indicate three distinct zones of induced earthquake activity that are consistent with the three clusters of seismicity identified by the company. The fault-plane solutions of 14 selected ML 0.6–1.8 events indicate a dominant reverse-faulting style, and the associated SH radiation patterns appear to control the first-order features of the macroseismic report distribution. Beamforming of earthquake data from six arrays suggests heterogeneous medium properties, in particular between the injection site and two arrays to the west and southwest. Ambient-noise cross-correlation functions reconstruct regional surface-wave propagation and path-dependent body-wave propagation. A 1D inversion of the weakly dispersive surface waves reveals average shear-wave velocities around 3.3 km/s below 20 m depth. Consistent features observed in relative velocity change time series and in temporal variations of a proxy for wavefield partitioning likely reflect the medium response to the stimulation. The resolution properties of the obtained data can inform future monitoring strategies and network designs around natural laboratories.

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A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - The seismicity of the Red Sea, Gulf of Aden and Afar triangle

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1970.0023 ◽

1970 ◽

Vol 267 (1181) ◽

pp. 49-74 ◽

Cited By ~ 93

Keyword(s):

Red Sea ◽

Body Wave ◽

Western Boundary ◽

Earthquake Activity ◽

Gulf Of Aden ◽

Transform Faults ◽

Fault Plane Solutions ◽

Afar Depression ◽

Sea Floor Spreading ◽

Afar Triangle

The seismicity of the Red Sea, Gulf of Aden and Afar triangle has been studied for the period January 1953 through December 1968. Epicentres have been relocated using the method of joint Epicentral Determination (Douglas 1967) and some fault plane solutions have been attempted. Magnitude-frequency studies indicate that with the present distribution of teleseismic stations, earthquakes with body wave magnitude m b ≽ 4.8 are well determined in this region. The study confirms that there is surprisingly little major earthquake activity in the northern part of the Red Sea. Between 19.5 and 21.0° N, there is a concentration of epicentres and some of these might be associated with an active NNE transform fault. In the southern part of the Red Sea, most of the epicentres are associated with the deep, axial trough, although some are associated with the western margin, especially in the neighbourhood of the Gulf of Zula (15° N). Earthquake activity is confined to the centre of the Gulf of Aden with concentrations of epicentres occurring on or near to NNE transform faults. The seismically active zone continues westwards through the Gulf of Tadjoura and across the Afar depression to the western boundary scarp. There are no teleseismically recorded epicentres between latitudes 12.2 and 14.2° N. In general, most of the seismic activity occurs along the centres of the Red Sea and Gulf of Aden and this supports a sea-floor spreading mechanism for their origin. The number of plates involved is discussed.

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SOURCE PARAMETERS OF MODERATE AND STRONG EARTHQUAKES IN THE BROADER AREA OF ZAKYNTHOS ISLAND (W. GREECE) FROM REGIONAL AND TELESEISMIC DIGITAL RECORDINGS

Bulletin of the Geological Society of Greece ◽

10.12681/bgsg.11391 ◽

2017 ◽

Vol 43 (4) ◽

pp. 2005

Author(s):

K. Chousianitis ◽

A. Agalos ◽

P. Papadimitriou ◽

E. Lagios ◽

K. Makropoulos

Keyword(s):

Epicentral Distance ◽

Signal To Noise Ratio ◽

Body Wave ◽

Moment Tensor ◽

Source Parameters ◽

Strike Slip ◽

Western Coast ◽

Fault Plane Solutions ◽

Waveform Data ◽

Good Signal

The source parameters of all the moderate and strong events that occurred in the broader area of Zakynthos Island for the period 1997–2009 are determined using two different techniques depending on epicentral distance. For the strong events we employed teleseismic body-wave modeling between 30° and 90°, while for moderate events a regional moment tensor inversion approach was used. In both cases we used broadband waveform data with a good signal-to-noise ratio. The calculated focal mechanisms are representative of the displacement and strain fields of the broader area. Those associated with the Cephalonia transform fault are consistent with dextral strike-slip motion. The area between the island of Zakynthos and the Western coast of Peloponnese is also characterized by strike–slip faulting, while reverse faulting is mainly observed south of Zakynthos Island. Using both techniques, we also retrieved the source parameters of the biggest events of the seismic sequence of April 2006 near the southern coast of Zakynthos Island. The depths of the located events of the sequence vary mainly between 10 and 25 km. The fault plane solutions revealed thrust type faulting in all cases, something which can be related with upward motions in the southern part of the Island observed by DGPS measurements.

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Guided Surface Waves on an Elastic Half Space

Journal of Applied Mechanics ◽

10.1115/1.3408973 ◽

1971 ◽

Vol 38 (4) ◽

pp. 899-905 ◽

Cited By ~ 5

Author(s):

L. B. Freund

Keyword(s):

Wave Propagation ◽

Surface Wave ◽

Surface Waves ◽

Half Space ◽

Body Wave ◽

Three Dimensional ◽

Elastic Half Space ◽

Normal Displacement ◽

Rigid Barrier ◽

Wave Disturbances

Three-dimensional wave propagation in an elastic half space is considered. The half space is traction free on half its boundary, while the remaining part of the boundary is free of shear traction and is constrained against normal displacement by a smooth, rigid barrier. A time-harmonic surface wave, traveling on the traction free part of the surface, is obliquely incident on the edge of the barrier. The amplitude and the phase of the resulting reflected surface wave are determined by means of Laplace transform methods and the Wiener-Hopf technique. Wave propagation in an elastic half space in contact with two rigid, smooth barriers is then considered. The barriers are arranged so that a strip on the surface of uniform width is traction free, which forms a wave guide for surface waves. Results of the surface wave reflection problem are then used to geometrically construct dispersion relations for the propagation of unattenuated guided surface waves in the guiding structure. The rate of decay of body wave disturbances, localized near the edges of the guide, is discussed.

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Strong earthquake activity influenced by solar flare intensity

10.5194/egusphere-egu21-3078 ◽

2021 ◽

Author(s):

Gerald Duma

Keyword(s):

Solar Flares ◽

Strong Earthquake ◽

Temporal Variations ◽

Seasonal Effect ◽

Earthquake Activity ◽

Strong Earthquakes ◽

International Service ◽

Earthquake Frequency ◽

Magnetic Index ◽

Magnetic Disturbances

Based on the comprehensive earthquake catalogue USGS ( HYPERLINK&#160; https://earthquake.usgs.gov) the paper demonstrates that strong earthquake activity, seismic events with M&#8805;6, exhibits a seasonal trend. This feature is the result of&#160; analyses of earthquake data for the N- and S- Earth Hemisphere in period 2010-2019. It can be shown also for single earthquake prone regions as well, like Japan, Eurasia, S-America.Any seasonal effect suggests an external influence. In that regard, one can think also of a solar-terrestrial effect, that is suggested already in several studies (e.g&#160; M.Tavares, A.Azevedo, 2011; D.A.E. Vares, M.A.Persinger,2014; G.Duma, 2019). This assumption leads to the question: Which dynamic process can cause a trigger effect for strong earthquakes in the Earth's lithosphere.In this study the intensity of solar flares and the resulting radiation, the solar wind, towards the Earth was taken into account. An appropriate parameter which has been regularity measured and reported for many decades and which reflects the intensity of solar radiation is the magnetic index Kp. It is measured at numerous geomagnetic observatories and describes the magnetic disturbances in nT within 3 hour intervals, respectively. Averages of all the measured 3-hour values are then published as Kp, therefore considered a planetary parameter (International Service of Geomagnetic Indices ISGI,France).The temporal variations of strong earthquake activity over 10 years and their energy release was compared with the above mentioned index Kp. Actually, a distinct correlation between the two quantities, Kp and earthquake frequency, resulted in cases of different regions as well as globally.&#160;Another essential result of the study is that maxima of Kp preceed those of earthquake activity by about 60 to 80 days in most cases. The mechanism has not yet been modeled satisfactorily.

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Aftershocks of the February 21, 1973 Point Mugu, California earthquake

Bulletin of the Seismological Society of America ◽

10.1785/bssa0660061931 ◽

1976 ◽

Vol 66 (6) ◽

pp. 1931-1952

Author(s):

Donald J. Stierman ◽

William L. Ellsworth

Keyword(s):

Fault Zone ◽

Main Shock ◽

Fault Plane ◽

Body Wave ◽

P Wave ◽

Fault System ◽

Wave Radiation ◽

Fault Plane Solutions ◽

Complex Deformation ◽

Transverse Ranges

abstract The ML 6.0 Point Mugu, California earthquake of February 21, 1973 and its aftershocks occurred within the complex fault system that bounds the southern front of the Transverse Ranges province of southern California. P-wave fault plane solutions for 51 events include reverse, strike slip and normal faulting mechanisms, indicating complex deformation within the 10-km broad fault zone. Hypocenters of 141 aftershocks fail to delineate any single fault plane clearly associated with the main shock rupture. Most aftershocks cluster in a region 5 km in diameter centered 5 km from the main shock hypocenter and well beyond the extent of fault rupture estimated from analysis of body-wave radiation. Strain release within the imbricate fault zone was controlled by slip on preexisting planes of weakness under the influence of a NE-SW compressive stress.

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Sierra Nevada-Great Basin boundary zone: Earthquake hazard related to structure, active tectonic processes, and anomalous patterns of earthquake occurrence

Bulletin of the Seismological Society of America ◽

10.1785/bssa0700051557 ◽

1980 ◽

Vol 70 (5) ◽

pp. 1557-1572

Author(s):

J. D. VanWormer ◽

Alan S. Ryall

Keyword(s):

Sierra Nevada ◽

Great Basin ◽

Temporal Variations ◽

Mono Lake ◽

Local Network ◽

Low Velocity ◽

Owens Valley ◽

Walker Lake ◽

Fault Plane Solutions ◽

The North

abstract Precise epicentral determinations based on local network recordings are compared with mapped faults and volcanic features in the western Great Basin. This region is structurally and seismically complex, and seismogenic processes vary within it. In the area north of the rupture zone of the 1872 Owens Valley earthquake, dispersed clusters of epicenters agree with a shatter zone of faults that extend the 1872 breaks to the north and northwest. An area of frequent earthquake swarms east of Mono Lake is characterized by northeast-striking faults and a crustal low-velocity zone; seismicity in this area appears to be related to volcanic processes that produced thick Pliocene basalt flows in the Adobe Hills and minor historic activity in Mono Lake. In the Garfield Hills between Walker Lake and the Excelsior Mountains, there is some clustering of epicenters along a north-trending zone that does not correlate with major Cenozoic structures. In an area west of Walker Lake, low seismicity supports a previous suggestion by Gilbert and Reynolds (1973) that deformation in that area has been primarily by folding and not by faulting. To the north, clusters of earthquakes are observed at both ends of a 70-km-long fault zone that forms the eastern boundary of the Sierra Nevada from Markleeville to Reno. Clusters of events also appear at both ends of the Dog Valley Fault in the Sierra west of Reno, and at Virginia City to the east. Fault-plane solutions for the belt in which major earthquakes have occurred in Nevada during the historic period (from Pleasant Valley in the north to the Excelsior Mountains on the California-Nevada Border) correspond to normaloblique slip and are similar to that found by Romney (1957) for the 1954 Fairview Peak shock. However, mechanisms of recent moderate earthquakes within the SNGBZ are related to right- or left-lateral slip, respectively, on nearly vertical, northwest-, or northeast-striking planes. These mechanisms are explained by a block faulting model of the SNGBZ in which the main fault segments trend north, have normal-oblique slip, and are offset or terminated by northwest-trending strike-slip faults. This is supported by the observation that seismicity during the period of observation has been concentrated at places where major faults terminate or intersect. Anomalous temporal variations, consisting of a general decrease in seismicity in the southern part of the SNGBZ from October 1977 to September 1978, followed by a burst of moderate earthquakes that has continued for more than 18 months, is suggestive of a pattern that several authors have identified as precursory to large earthquakes. The 1977 to 1979 variations are particularly noteworthy because they occurred over the entire SNGBZ, indicating a regional rather than local cause for the observed changes.

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Design and Validation of Probes and Sensors for the Characterization of Magneto-Ionic Radio Wave Propagation on Near Vertical Incidence Skywave Paths

Sensors ◽

10.3390/s19112616 ◽

2019 ◽

Vol 19 (11) ◽

pp. 2616

Author(s):

Ben A. Witvliet ◽

Rosa M. Alsina-Pagès ◽

Erik van Maanen ◽

Geert Jan Laanstra

Keyword(s):

Magnetic Field ◽

Wave Propagation ◽

Radio Wave ◽

Dynamic Range ◽

Signal To Noise Ratio ◽

Radio Wave Propagation ◽

Direct Digital Synthesis ◽

Earth’S Magnetic Field ◽

Earth's Magnetic Field ◽

Propagation Of Waves

This article describes the design and validation of deployable low-power probes and sensors to investigate the influence of the ionosphere and the Earth’s magnetic field on radio wave propagation below the plasma frequency of the ionosphere, known as Near Vertical Incidence Skywave (NVIS) propagation. The propagation of waves that are bent downward by the ionosphere is dominated by a bi-refractive mechanism called ‘magneto-ionic propagation’. The polarization of both downward waves depends on the spatial angle between the Earth’s magnetic field and the direction of propagation of the radio wave. The probes and sensors described in this article are needed to simultaneously investigate signal fading and polarization dynamics on six radio wave propagation paths. The 1-Watt probes realize a 57 dB signal-to-noise ratio. The probe polarization is controlled using direct digital synthesis and the cross-polarization is 25–35 dB. The intermodulation-free dynamic range of the sensor exceeds 100 dB. Measurement speed is 3000 samples/second. This publication covers design, practical realization and deployment issues. Research performed with these devices will be shared in subsequent publications.

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A functional tool to explore the reliability of micro-earthquake focal mechanism solution for seismotectonic purposes

10.5194/se-2021-88 ◽

2021 ◽

Author(s):

Guido Maria Adinolfi ◽

Raffaella De Matteis ◽

Rita De Nardis ◽

Aldo Zollo

Keyword(s):

Focal Mechanism ◽

Fault Plane ◽

Focal Mechanisms ◽

Seismic Network ◽

Fault System ◽

Focal Mechanism Solution ◽

Fault Plane Solutions ◽

Stress Regime ◽

Focal Mechanism Solutions ◽

Earthquake Focal Mechanism

Abstract. Improving the knowledge of seismogenic faults requires the integration of geological, seismological, and geophysical information. Among several analyses, the definition of earthquake focal mechanisms plays an essential role in providing information about the geometry of individual faults and the stress regime acting in a region. Fault plane solutions can be retrieved by several techniques operating in specific magnitude ranges, both in the time and frequency domain and using different data. For earthquakes of low magnitude, the limited number of available data and their uncertainties can compromise the stability of fault plane solutions. In this work, we propose a useful methodology to evaluate how well a seismic network used to monitor natural and/or induced micro-seismicity estimates focal mechanisms as function of magnitude, location, and kinematics of seismic source and consequently their reliability in defining seismotectonic models. To study the consistency of focal mechanism solutions, we use a Bayesian approach that jointly inverts the P/S long-period spectral-level ratios and the P polarities to infer the fault-plane solutions. We applied this methodology, by computing synthetic data, to the local seismic network operated in the Campania-Lucania Apennines (Southern Italy) to monitor the complex normal fault system activated during the Ms 6.9, 1980 earthquake. We demonstrate that the method we propose can have a double purpose. It can be a valid tool to design or to test the performance of local seismic networks and more generally it can be used to assign an absolute uncertainty to focal mechanism solutions fundamental for seismotectonic studies.

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Investigation of ambient noise seismological methods on a bridge model

10.5194/egusphere-egu21-2773 ◽

2021 ◽

Author(s):

Chun-Man Liao ◽

Franziska Mehrkens ◽

Celine Hadziioannou ◽

Ernst Niederleithinger

Keyword(s):

Wave Propagation ◽

Large Scale ◽

Seismic Velocity ◽

Measurement Data ◽

Correlation Coefficients ◽

Coda Wave ◽

Velocity Change ◽

Promising Tool ◽

Bridge Model ◽

Noise Measurements

The aim of this work is to investigate the application of seismological noise-based monitoring for bridge structures. A large-scale two-span concrete bridge model with a build-in post-tensioning system, which is exposed to environmental conditions, is chosen as our experimental test structure. Ambient seismic noise measurements were carried out under different pre-stressed conditions. Using the seismic interferometry technique, which is applied to the measurement data in the frequency domain, we reconstruct waveforms that relate to wave propagation in the structure. The coda wave interferometry technique is then implemented by comparing two waveforms recorded in two pre-stress states. Any relative seismic velocity changes are identified by determining the correlation coefficients and reveal the influence of the pre-stressing force. The decrease of the wave propagation velocity indicates the loss of the pre-stress and weakening stiffness due to opening or event extension of cracks. We conclude that the seismological methods used to estimate velocity change can be a promising tool for structural health monitoring of civil structures.

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Variations in noise and signal levels in a pair of deep boreholes near Amarillo, Texas

Bulletin of the Seismological Society of America ◽

10.1785/bssa0840051593 ◽

1994 ◽

Vol 84 (5) ◽

pp. 1593-1607

Author(s):

Christopher J. Young ◽

Eric P. Chael ◽

David A. Zagar ◽

Jerry A. Carter

Keyword(s):

Noise Level ◽

Signal To Noise Ratio ◽

Body Wave ◽

Working Hours ◽

Data Set ◽

Frequency Independent ◽

Deep Boreholes ◽

Strong Body ◽

Borehole Seismic ◽

40 Hz

Abstract To better understand the depth dependence of improvements in signal-to-noise ratio for borehole seismic data, we have collected and analyzed a data set recorded from a pair of high-fidelity, broadband (1 to 80 Hz) seismometers sited in two closely separated deep boreholes near Amarillo, Texas. The total decrease (surface to 1951 m) in minimum noise level is up to 30 dB; the total decrease in maximum noise level is up to 35 dB; and the range of noise values (maximum to minimum) at a given depth decreases from 30 to 10 dB. The majority of the noise reduction occurs within the first few hundred meters below the surface, but the decrease continues with depth and there is no indication that even lower noise levels do not exist below our deepest recording depth (1951 m). Cultural work-day noise (prominent during normal working hours) from 0 to 40 Hz is observed at all depths, suggesting a strong body-wave component. Wind-generated 15 to 60-Hz noise is strongest at the surface, and is observed as deep as 367 m without a properly shielded hole. With the addition of shielding to reduce the coupling between the borehole casing and/or cable with the wind, wind-generated noise above 40 Hz can be eliminated at 367 m. Events with near-vertical propagation paths and signal power above 1 Hz show signal strength decreasing uphole, with a steady winnowing of high frequencies that can be fit by a simple frequency-independent Q of 35.5 ± 8 for the material between 1951 and 367 m.

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